Heat flux measurement pipe and method for determining sprinkler water delivery requirement

ABSTRACT

A method of evaluating fire hazards of materials in standard tests is provided which comprises measuring heat flux distribution in test fires at the moment when sprinklers would have sensed the fire and be activated, and determining the rate of sprinkler water delivery rate that will absorb the heat flux, thus controlling spread of the fire.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for determining a sprinkler waterdelivery requirement to control a fire. The invention also relates to anapparatus for measuring heat flux, especially in connection with fireprotection, the fire testing of materials, and determining a sprinklerwater delivery rate.

2. Background of the Invention

The fire hazard represented by storage of a given material is oftencharacterized by the rate of delivered sprinkler water required tosuppress or control a fire of that material. The presentstate-of-the-art for hazard characterization is to perform replicatetests in which: the material is ignited; the fire is allowed to growuntil it is sensed by sprinklers; and the sprinklers then activate todeliver water to the fire. The delivered water density, that is, theamount of water delivered by the sprinklers per unit area of the floor,is systematically increased until a delivery rate that controls the fireis found. Many standard tests must be performed with the same materialto determine what rate of sprinkler water delivery is required tocontrol the fire from the burning material. These replicate testsconsume a great deal of personnel labor and material, and are thus veryexpensive and time-consuming.

The severity of fires and the hazards they present are assessed in termsof the total chemical heat release rate of the fire and the heat fluxemitted. Heat flux is defined as the rate of energy transfer per unitsurface area. Heat flux is typically expressed in units of kilowatts persquare meter or BTU per square foot per minute. The measurement of heatflux is of importance in many sciences, including the fire testing ofmany materials. The heat flux emitted by burning materials may ignite,or aid in the burning of, nearby materials. In one known test set up, agas burner is positioned at the base of and between two parallel panelson which a test material, for example, a fire resistant material such aspolyurethane insulation, is placed. Measurement of heat flux in thisparallel panel test provides valuable information about the response ofthe test material to the flames from the burner.

Instrumentation presently available for measuring heat flux requirescomplex, time-consuming installation, and is not sufficiently robust towithstand repeated use in very severe fire environments. Theconventional instrumentation usually consists of water-cooled heat fluxgauges that need to be individually installed, for example, directly onthe panels bearing the test material. These gauges are exposed to flamesduring use. Individual heat flux gauges must undergo time-consumingcalibration before and after the test because their sensing elements areeasily damaged or impacted by fire impingement and by the deposition ofsoot and other incomplete products of combustion. A measurementuncertainty arises when post-test calibration shows that the gaugecalibration constant has shifted as a result of this impact. Moreover,the gauges are individually water-cooled and mounted to view the flamesthrough openings drilled in the material and supporting structure. Thisadds time and expense to the testing program and severely limits thenumber of heat flux measurement stations that can be installed. In somefire test configurations, such as commodity classification, it is notpractical to install heat flux gauges due to the difficulty ofprotecting water cooling lines and electrical connections in highlyhazardous locations.

SUMMARY OF THE INVENTION

An object of the invention is to provide an inexpensive, easilyinstalled device for measuring heat flux distribution.

Another object of the invention is to provide a simple method formeasuring heat flux distribution.

Yet another object of the invention is to provide a durable apparatusfor measuring heat flux distribution from gas burners or fire testingapparatuses.

Still another object of the invention is to provide a method forevaluating fire hazards based on measurements of the heat flux in testfires.

Another object of the invention is to provide a method for evaluatingthe total heat transfer to the burning fuel from spatially distributedheat flux measurements. The total heat transfer is defined as theproduct of heat flux and the area receiving that heat flux, summed overthe entire area receiving heat flux. Total heat transfer is typicallyexpressed in units of kilowatts or BTU/minute.

A further object of the invention is to provide a method for determiningthe area over which heat is transferred to the material.

Another object of the invention is to provide a method for determiningthe rate of flow of sprinkler water required to control a fire based ona measurement of the total heat transfer to the burning fuel.

A further object of the inventions is to have a single test that is ableto determine the required flow rate of sprinkler water necessary tocontrol a fire for a given material.

The amount of sprinkler water flow rate required to control a fire canbe determined by the method of the present invention which includes:measuring the spatial heat flux distribution in a test fire; calculatingthe effective heat flux received by the material surface, andcalculating the sprinkler water delivery rate needed to absorb the heatflux using the energy required to vaporize the delivered water.

The method of the present invention solves the problems of conventionalmethods by reducing or eliminating the need for multiple and incrementaltesting. It has been discovered that the sprinkler water delivery raterequired for control of the commodity is proportional to the total heattransfer to the fuel (i.e. product of the flame heat flux and flamearea) just before the moment when sprinklers sense the fire, causing thesprinkler valve to open and deliver water to the fire. Theproportionality constant is easily calculated from the heat ofvaporization of water, that is, the rate at which water will beconverted to steam per unit of applied heat flux.

By the present invention, the amount of sprinkler water necessary tocontrol the burning of a material can be determined from a single test.The method of the present invention enables an evaluation of the firehazard of materials based on heat flux measurements. The rate ofsprinkler water required to control an array of a burning commodity,such as a commodity in a warehouse, is proportional to the heat flux tothe surface of the commodity. The heat flux transferred to the heat fluxmeasurement pipe of the present invention in a free-burning fire isclosely related to the water flow rate required to suppress the fire.

Instead of individually installed heat flux gauges fixed to test panelsto measure heat flux at various heights in a fire test, the heat fluxmeasurement pipe, or device, of the present invention is a unitarydevice that has the capability for simultaneous measurements of heatflux along its length. The heat flux measurement pipe of the presentinvention is extremely stable and rugged, has no moving parts, and iseasy to position in a test set up. The heat flux measurement pipe doesnot need to be connected to the panels bearing the test material.Instead, it can merely be positioned near or between the panels while,for example, being supported on wheeled support.

The heat flux measurement pipe is a water-cooled pipe that makes use ofthe change in water temperature over a distance along a waterpassageway, for example, a spiral water passageway, within the pipe. Anouter pipe fits tightly over an inner core into which a spiral waterpassageway is machined. Thermocouples for measuring the temperature ofthe water in the water passageway are fixed on the core at spacedlocations in the water passageway, adjacent thermocouples definingsections of the water passageway between them. At steady-state, the netheat transfer rate to each section of the water passageway can bedetermined from the product of mass flow rate of water entering orleaving that section and the difference in water temperature between theentrance and exit of that section. Accordingly, only the overall watermass-flow-rate and the water temperature at various locations along thepassageway need be measured to determine the spatial distribution ofheat transferred to the pipe. The thermocouples measure the temperatureof the water at the entrance and exit of each section of the waterpassageway. By forcing the water to travel in a tight spiral in anannulus between the outer pipe and the core, there is assurance that anyspatial non-uniformity will be averaged out, improving heat transfer andresulting in a water temperature that represents the average heattransfer to each pipe section. Heat flux levels can be readilydetermined from the measured water temperatures and the pipe geometryusing known calculations.

The heat flux measurement pipe of the present invention can providemeasurements of the heat flux distribution from flames in standard teststo evaluate the fire hazard of typical warehouse commodities anddetermine the rate of sprinkler water delivery needed to control thefire.

Thus, the use of the heat flux measurement pipe in fires involving testmaterials measures the spatial distribution of heat flux, and from thisthe sprinkler water delivery rate that would absorb the heat flux inconverting the water to steam can be determined. When the heat flux fromthe fire is absorbed, the fire will stop spreading and burn out. Thismethod of evaluating fire hazards determines the required sprinklerwater delivery rate from heat flux measurements of a single test, ratherthan from the multiple tests that were heretofore considered necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention is explained in greater detail by way ofthe drawings, where the same reference numerals refer to the samefeatures.

FIG. 1 is a schematic perspective view of a heat flux measurement pipeaccording to the present invention in position at a fire test set up.

FIG. 2 is a cross section of the heat-flux measurement pipe of FIG. 1with a portion moved.

FIG. 3 is a schematic illustration of an arrangement of temperaturesensors in the heat-flux measurement pipe of FIG. 1.

FIG. 4 is a schematic illustration of a fire test set up employing heatflux measurement pipes according to the present invention.

FIG. 5 is a graphical representation of the vertical and horizontaldistribution of heat flux of the fire in the fire test set up of FIG. 4.

FIG. 6 is a sketch showing how to determine the effective heat flux andlength from the measured vertical and horizontal heat flux distributionsshown in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As can be seen from FIG. 1, the heat flux measurement device, or heatflux measurement pipe, according to the present invention, which isdesignated generally by the reference numeral 1, is designed to beinstalled within a testing apparatus 4 for testing the flame propertiesof materials. The heat flux measurement device 1 runs the verticallength of a testing apparatus 4 that includes a gas burner 2 and testpanels 6 of material to be tested. Cold water enters the heat fluxmeasurement device 1 through a water flow rate measurement device 3 andan inlet conduit 8 connected to the top of the device, and heated waterexits through an outlet conduit 10 connected at the bottom of thedevice. The product of the water flow rate and the segmental increasesin water temperature as water flows through the device 1 indicates thedistribution of heat transfer to the device 1 from both the flames 5being supplied by the gas burner 2 and the panels 6 of the materialbeing tested. From this heat transfer distribution, information aboutthe fire behavioral characteristics of the panels 6 can be obtained.

As can be seen from FIG. 2, the heat flux measurement device 1 has acore 12 that can be generally cylindrical and made out of aluminum. Thecore 12 is machined to form a spiral groove, which defines areduced-diameter portion 20 and extends the length of the core. Themachining leaves on core 12 a helical rib 16 that snugly engages theinside of an outer pipe 18 defining the exterior of a heat fluxmeasurement device 1. The outer pipe 18 can be made of steel. Thereduced diameter portion 20 defines a radially inner surface of a spiralwater passageway 14. The inlet conduit 8 (FIG. 1) is connected to thetop of the passageway 14, and the outlet conduit 10 (FIG. 1) isconnected to the bottom of the passageway 14. Typically, the core outerdiameter is about 0.74 inches; the distance between convolutions of therib is about 0.5 inches; the rib 16 is about 0.125 inches wide; and thedistance between the inner surface of the outer pipe 18 and thereduced-diameter portion 20 of the core 12 is about 0.135 inches.Thermocouple lead wires 24 are shown extending out the top and bottom ofthe core. Portions of the lead wires 24 extending through the passageway14 are not shown.

As can be seen from FIG. 2, thermocouples 22 a-22 g are located atspaced positions along the spiral water passageway 14 of the core. Aschematic of the thermocouple layout is shown in FIG. 3. Thethermocouple leads are threaded helically around the core 12 through thepassageway 14 to the exit of the passageway at the bottom of the heatflux measurement device 1 and then to a measurement station wherethermocouple differential voltages are measured. Generally, athermocouple is a sensing element with two dissimilar metallicconductors joined end to end, the junction, when heated, producing avoltage differential between the two conductors. The temperature at thethermocouple lead can be determined from the voltage differential usingstandard tables. In the present invention, the thermocouples 22 a-22 gare used to measure the temperatures of the water flowing through theheat flux measurement device 1 at their respective locations. Theproduct of the water flow rate and the temperature difference betweenadjacent thermocouples determines is the heat gain in the segment ofpipe between these thermocouples. Dividing this heat gain by the surfacearea of the pipe between the adjacent thermocouples gives the local heatflux.

Lead wires 24 from the thermocouples 22 a-22 g extend upward through thepassageway 14 and out at the top of the device 1. For this purpose, aport can be formed adjacent to the inlet of the passageway 14, throughthe top of the inner core 12, with the lead wires extending through anelastomeric plug in the port in a watertight manner. Alternativearrangements for extending the lead wires 24 out of the device can bemade. In the illustrated embodiment, lead wires 24 to each of thethermocouples 22 a-22 g extend down from the top of the heat fluxmeasurement pipe device 1 through the spiral water passageway 14. Thelead wires to the various thermocouples 22 a-22 g have different lengthsso as to suspend the junctions of the thermocouples at various positionsin the spiral passageway 14, the junctions being spaced axially from oneanother along the axis of the heat flux measurement device 1. The heatflux measurement device 1 has the seven thermocouples 22 a-22 g spacedat distances of 0.5, 1.5, 2.5, 3.5, 5.5, 7, and 9 feet, respectively,measured from the bottom. This spacing provides greater resolution inthe region of the fire. The thermocouples 22 a, 22 f , and 22 g areshown in FIG. 2 in exemplary positions along the spiral passageways 14.The other thermocouples are not shown in FIG. 2. Other spacingarrangements can also be useful. The thermocouples 22 a-22 g can beungrounded junction chromel-alumel thermocouples. The ends of the leadwires 24 opposite to the junctions of the thermocouples 22 a-22 g areconnected to devices for recording the temperatures and/or calculatingheat fluxes in accordance with known formulas.

The water flow rate through the pipe is measured by an electronicflowmeter 3 (shown in FIG. 1), such as the +GF+Signet 8511 Micro FlowSensor. This device is mounted on the water inlet 8 to the heat fluxmeasurement device 1 as is shown in FIG. 1. Leads (not shown) from theflowmeter are routed to the instrumentation station, where thermocouplesignals and water flow rates are simultaneously recorded. The heat fluxto a section of the heat flux measurement device 1 is the product of thewater flow rate, the specific heat of water (expressed in units such asjoules per gram degree C or BTU per pound degree F) and the differencein water temperature entering and exiting the section, respectively,divided by the exterior surface area of this section of the pipe. Theseven thermocouple locations in the embodiment shown in FIG. 1 thusprovide six heat flux measurements at distances along the pipecorresponding to the midpoints between pairs of adjacent thermocouples.The respective heat fluxes are measured simultaneously with acomputer-based data acquisition system Calibration of this devicerequires only an initial calibration of the water flow measuring device3 and a minor correction for differences in thermocouple offsetvoltages, which is easily deduced from pre-test heat flux measurements.

The heat flux measurement device 1 has a small diameter so that it doesnot disturb the fire or the airflow near the fire, thus ensuring thatthe device itself does not alter the measurement of flame heat flux. Thediameter of the annulus and the flow rate of water are chosen to: (1)ensure efficient heat transfer from the pipe to the water-immersedthermocouples; (2) ensure that temperature differences betweenthermocouples are large enough to be accurately measured, but not solarge as to cause boiling before the water exits the pipe; (3) ensurethat the heat flux measurement will be responsive to transient firebehavior; and (4) ensure a reasonable water pressure drop across thepipe.

The heat flux measurement device 1 is manufactured to be a ruggedinstrument that is easy to install and calibrate, thus dramaticallyreducing the time and effort involved in installation in fire testconfigurations. It be easily positioned in most existing large-scalestandard fire tests. The device 1 is water-cooled and highly durable,having no sensing elements that are exposed to flames.

FIG. 4, FIG. 5 and FIG. 6 illustrate a measurement of both the spatialextent of heat flux from a fire and the evaluation of the total heattransfer to the test material, by using the method according to thepresent invention. In FIG. 4, a test material 31 is mounted in a wallconfiguration and ignited, followed by flame spread on the wall surface,32. Heat pipes 33 and 34 adjacent to the wall provide the horizontal 35and vertical 36 distributions of the heat flux from the fire as isillustrated by FIG. 5. Alternatively, the method of the presentinvention can be carried out by using linear arrays ofheat-flux-gauges-instead of the heat pipes 33 and 34. The area undereach curve represents the integral of the heat flux with respect todistance along the horizontal 33 or vertical 34 heat pipe. The totalheat transfer to a burning commodity is calculated by integrating inboth the horizontal and vertical directions. The heat flux measurementsalso indicate the spatial extent of the fire, horizontally 35 a-35 b,and vertically, 36 a-36 b, from which the burning surface area isinferred.

In order to simplify the calculation of the total heat transfer to thetest material, the measured horizontal or vertical distributions of heatflux is replaced with an effective heat flux and effective width (orheight). FIG. 6 shows how to calculate the effective heat flux, q₀, andthe effective width, l, from a measured heat flux distribution. Theeffective heat flux, q₀, and the effective width, l, of the heat fluxcan be determined by first setting

lq ₀=∫_(∞) ^(∞) f(x)dx

and then choosing q₀ and l that minimize the integral

B=∫ _(∞) ^(∞) [f(x)−q(x)]² dx

Here f(x) is the measured variation in heat flux with distance, x andq(x) is the variation in the effective heat flux q₀ with distanceq(x)=q₀ over the width (or height), l, and is equal to zero outside thiswidth (or height). The procedure can apply to either the vertical orhorizontal distributions.

The density of water per unit material surface area, m_(w) ^(n), neededto absorb the heat flux, q₀, is equal to q₀/L, where L is the heatrequired to vaporize a unit mass of the sprinkler water. Fire protectionengineers express the required sprinkler water flow rate, D, in terms ofthe sprinkler water flow rate per unit floor area, in units of mm/minuteor gallons per square foot per minute. To calculate D, the amount ofexposed material surface area per unit floor area is required. Thus, leta_(f) be the ratio of the material surface area divided by theassociated floor surface area. In a fire test configuration comprisingvertical and horizontal material surfaces, one is usually most concernedwith the vertical surfaces. Using this definition, the sprinkler waterflow rate that is needed to control the fire is$D = {{{\overset{.}{m}}_{w}^{''}a_{f}} = \frac{q_{0}a_{f}}{L}}$

where a_(f) is the area of the exposed material per unit floor area. Itis assumed here that the sprinkler water can reach most of the exposedtest material surface area. In addition, it is assumed that the water isapplied soon enough for it to reach the burning surface without beingblown away by the rising fire plume from a fire that has grown to becomevery large.

Test materials are often classified in terms of their relative firehazard, or the sprinkler water flow rate per unit surface area requiredto control the fire at the moment when sprinklers might sense the fireand activate.

Although the invention is described in detail with respect to apreferred embodiment, it will now be apparent from the foregoing tothose skilled in the art that changes and modifications may be madewithout departing from the invention in its broader aspects, and theinvention, therefore, as defined in the claims is intended to cover allsuch changes and modifications as fall within the true spirit of theinvention.

What is claimed is:
 1. A method of determining a sprinkler water flow rate from at least one sprinkler required to control a fire comprising: measuring spatial heat flux distribution to a burning material in a test fire; calculating the effective heat flux, q₀, to the burning material; and calculating a sprinkler water delivery rate per unit exposed material area, m_(w) ^(n)=q₀/L, required to absorb the heat transfer through a vaporization of water, wherein L is the heat required to vaporize a unit mass of water.
 2. The method of claim 1, wherein the method further comprised estimating a sprinkler water flow rate per unit floor area, D=m_(w) ^(n)a_(f)=q₀a_(f)/L, required to control a fire of the material, wherein a _(f) is the area of exposed material per unit floor area.
 3. The method of claim 1, wherein said step of measuring the heat flux distribution comprises measuring the flow rate of a fluid flowing through a passageway of an elongated element; and measuring the temperature of the fluid at points along the passageway spaced longitudinally from one another with respect to the passageway.
 4. The method of claim 3, wherein the step of measuring the flow rate comprises measuring the flow rate through a spiral passageway between an outer pipe and an inner core within the outer pipe.
 5. The method of claim 3, wherein the step of measuring the temperature comprises extending lead wires with temperature-sensing elements longitudinally through the passageway to the top of the elongated element.
 6. The method of claim 3, wherein the step of measuring the flow rate comprises measuring the flow rate of a fluid flowing through a spiral passageway extending axially within the elongated element.
 7. The method of claimed 3, wherein said step of measuring the flow rate comprises measuring the flow rate through a spiral passageway defined at least in part by a helical rib on an inner pipe within an outer pipe.
 8. The method of claim 7, wherein the step of measuring the flow rate comprises measuring the flow rate through a passageway defined at least in part by an outer pipe having a diameter of less than 1.0 inches.
 9. The method of claim 7, wherein the step of measuring the temperature comprises extending lead wires of temperature-sensing elements longitudinally through the passageway to the top of the elongated element. 